Much audio equipment is sold as separate components, for example, phonograph and compact disc players, amplifiers, and loudspeakers, and these components need to be connected to each other by cable. The cable needs to be flexible, so that the user may arrange his components as he wishes; cables should also, at least to some extent, be interchangeable, and to that end certain standard connectors are provided on both components and cables.
These cables fall into two basic categories: small-signal cables, carrying low-voltage, low-current signals (up to 2 volts at 250 milliamps or so), for example between phonograph and preamplifier, and between preamplifier and power amplifier; and large-signal cables, carrying up to several hundred watts, which applies only to connections between power amplifier and loudspeakers, and in rare cases to connections between sections of a loudspeaker/crossover network when the network is exposed. Both of these kinds of cables are usually single-ended dipole cables, that is, they consist of two conductors, one carrying the positive signal and the other carrying both the negative signal and ground. The small-signal cables are usually shielded against radio-frequency interference and external hum fields.
Each of these kinds of cable has a conventional termination. Small-signal cables and the components to which they are connected use a mating male/female pair (male on cable, female on component) of the type commonly called "RCA" connectors. Large-signal cables are generally terminated either with bare wire or with spade lugs, and these terminations are fastened to amplifiers and loudspeakers with binding posts or other simple screwed or spring-loaded terminals.
Research and experience have shown that the cable used between components can have a significant impact on the overall sound of a system of high-fidelity components. For that reason manufacturers and audiophiles seek electrically optimal cables, cables adding the least possible distortion and coloration of their own to the musical signal as it passes through an audio system. Many types of distortion are recognized, although it is not always known why or how a certain kind of signal distortion produces a certain acoustic or psycho-acoustic effect.
Most audio cable is composed of bundles of stranded wires in variously twisted and braided configurations. At least two kinds of distortion are, however, inherent in stranding. Microphony, or electrically induced motion among the strands, produces spurious induction among the strands, which is known empirically to blur the mid-upper frequency audio range. Secondly, small-diode or rectification effects arise from imperfect contact among the strands, producing harshness audible in the higher frequencies of the audio bandwidth.
Further, skin effect is difficult to eliminate from stranded wire designs. Skin effect occurs when signals of different frequencies travel at different depths in a conductor--higher frequencies penetrate less deeply--and thus undergo relative phase shift, which distorts the time-coherency of the audio signal. There are studies which suggest that the techniques used to eliminate skin effect in stranded-wire designs--such as using very fine wires to maximize conductor surface area--do not in fact eliminate skin effect from the uppermost octave of the audio band.
Most cables' conductors are enclosed within some kind of plastic insulator or dielectric, which serves in part to hold stranded bundles together. Because of their contact with the conductor strands, these solid dielectrics compound the problem of microphony. Solid dielectrics also increase a cable's capacitance by energy storage effects, which, especially in a long cable run, can create stability problems in wide-band audio electronics.
Low resistance is important in cables used between phonograph and preamplifier where low source impedance moving-coil cartridges are used; it is also important in cables used between power amplifiers and loudspeakers, since the resistance connecting cable compromises the amplifier's damping factor. Thus to counter resistance, the conductors of many cables are made large in cross-section.
Along with the conductor's increased diameter, however, comes increased internal inductance, causing phase shift or even amplitude roll-off in the higher frequencies of the audio band. If positive and negative conductors are brought close together, their external (also called mutual) inductance counters the internal inductance, but this close spacing of the conductors also gives rise to increased capacitance in the cable. Conductors of large diameter also exhibit increased skin effect.
In their efforts to overcome these problems, designers use an enormous variety of materials and methods, tuning out distortions one by one. One popular and simple attempt at purity is the use of "solid-core" wire, or individual strands (one per pole) of uninsulated copper wire from 0.5 to 1 mm in diameter. Such a small diameter in a conductor evades skin effect, but increases resistance. If the skin effect is accepted in a strand made heavier for low resistance, inductance increases. If the internal inductance is countered by external inductance, capacitance increases--so while leaving the problems of stranding and insulators behind, solid-core users still carry heavy loads of electrical trade-offs.
A further signal distortion in conventional cables arises from their connectors. These connectors are usually made of metals less conductive than copper, and further are attached to cables either with solder--itself a poor conductor--or by compression fittings. Both compression fittings and solder joints are subject to failure.
One approach to optimizing certain electrical characteristics that has been used in audio cable is the simulation of a tubular conductor by means of stranded wires, usually surrounding a slender non-conductive cylindrical former, for example as the outer conductor in coaxial configurations. While this technique does reduce the inductance of the wire bundle versus the same bundle formed into conventional conductor of twisted strands, the other problems of stranded wire mentioned above remain.
Solid-wall hollow structures made of copper have been used in microwave and other high-frequency technology. Solid-walled hollow structures of both rectangular and circular cross-section are used as waveguides for microwave transmissions; although the structures also conduct current, their dimensions are chosen for their waveguide capabilities. Solid-wall hollow conductors are used in some high-frequency applications in fixed positions on circuit boards, in part to permit coolant to be passed through them. Hollow structures having extremely thin walls, for example of deposited film, are believed to have been used as conductors of very high frequencies. Tubular conductors having segmented walls are used in high-power transmission.
Because of size, lack of flexibility, or lack of durability, none of these hollow conductors lends itself to use in cable for transmission of audio-frequency signals.